1. Field of the Invention
The present invention relates to internal combustion engine control technologies, and more particularly to an internal combustion engine control technology suitable for minimizing the degree of combustion status variation during idling.
2. Background Art
While an internal combustion engine is idle, combustion status varies from one cylinder to another. Therefore, torque variation is likely to occur in the internal combustion engine. Torque variation not only makes the idle revolving speed unstable, but also incurs vibration. A known technology disclosed, for instance, by Japanese Patent No. 2505304 (hereinafter referred to as “Patent Document 1”) is used to minimize the degree of torque variation during idling.
The technology disclosed by Patent Document 1 detects the revolution variation of each cylinder during idling. If the magnitude of the revolution variation of a certain cylinder exceeds an upper limit, this technology decreases the injection amount for that cylinder and increases the injection amounts for the other cylinders. If, on the other hand, the magnitude of the revolution variation of a certain cylinder is below a lower limit, the technology increases the injection amount for that cylinder and decreases the injection amounts for the other cylinders. If the increment of the injection amount exceeds a limit when the injection amount is increased, the technology advances the ignition timing. If the decrement of the injection amount is lower than a limit when the injection amount is decreased, the technology delays the ignition timing.
It is conceivable that the above conventional technology produces its effects when a certain degree of combustion variation is caused, for instance, by intake distribution differences amount the cylinders during idling after warm-up.
During idling, particularly cold fast idling immediately after a cold start, the air-fuel ratio is make lean from the viewpoint of fuel efficiency. Further, the ignition timing is delayed from the viewpoint of fuel efficiency and from the viewpoint of catalyst warm-up for exhaust emission control. Consequently, extremely unstable combustion results. Experiments conducted by the inventors of the present invention have revealed that a phenomenon not predicted by the above conventional technology is encountered during cold fast idling during which combustion is extremely unstable.
FIG. 1 is a graph illustrating torque changes of a specific cylinder that occur when no special control is exercised during cold fast idling. In FIG. 1, the torque region between two threshold values (upper and lower threshold values) represents a proper torque region for an idling period. The region above the proper torque region is a fast combustion region, whereas the region below the proper torque region is a partial combustion region. Partial combustion refers to combustion during which no high torque is generated. Fast combustion refers to a combustion during which high torque is generated. In FIG. 1, cycles in the proper torque region are marked by a white circle, whereas cycles outside the proper torque region are marked by a black circle. When the torque decreases due to partial combustion, fast combustion occurs in the affected cylinder in the next cycle so that the torque suddenly increases as indicated in the figure. When, on the other hand, the torque increases due to fast combustion, partial combustion occurs in the affected cylinder in the next cycle so that the torque suddenly decreases.
In partial combustion, the exhaust temperature rises by the amount of fuel energy that is not used for torque generation. Therefore, fast combustion occurs in a cycle next to a partial combustion cycle probably because the ratio of residual gas lowers in the next cycle or the unburned gas temperature rises in the next cycle. It is also conceivable that fast combustion occurs in the next cycle because the equivalent ratio increases in the next cycle due to frequent residual hydrocarbon generation. Partial combustion occurs in a cycle next to a fast combustion cycle for a reason that is contrary to the above-mentioned one. In any case, partial combustion and fast combustion alternately occur during cold fast idling, thereby incurring torque variation.
When the above conventional technology is used, the fuel injection amount, ignition timing, and other control parameters are adjusted so as to decrease the torque of a cylinder if it has a high torque or increase torque of a cylinder if it has a low torque. Therefore, if control is exercised according to the above conventional technology while partial combustion and fast combustion alternately occur as shown in FIG. 1 and if the torque is increased due to fast combustion, the control parameter setting for the next cycle is adjusted to decrease the torque. Therefore, partial combustion in the next cycle is promoted. If, on the contrary, the torque is decreased due to partial combustion, the control parameter setting for the next cycle is adjusted to increase the torque. Therefore, fast combustion in the next cycle is promoted. In other words, even if control is exercised according to the above conventional technology during cold fast idling, combustion status variation cannot be effectively reduced, but the degree of combustion status variation is increased. Combustion status variation may not only incur torque variation to decrease drivability, but also degrade the exhaust gas performance.